1. Field of the Invention
The present invention relates to image pickup apparatuses, image pickup methods, exposure control methods, and programs, and, more particularly, to exposure correction in which an image pickup apparatus generates a combined image signal with a wide dynamic range by combining a long-exposure image signal and a short-exposure image signal.
2. Description of the Related Art
In many image pickup apparatuses using a solid-state image pickup device such as a CCD (Charge Coupled Device), the quantity of light to be input into the image pickup device (light quantity of exposure) is controlled by controlling an aperture and an electronic shutter speed. That is, when a bright scene is captured, the light quantity of exposure is reduced so as to prevent the occurrence of overexposure that is caused by saturation of a signal output from an image pickup device. Conversely, when a dark scene is captured, the light quantity of exposure is increased so as to prevent the occurrence of underexposure.
However, when a scene with a strong contrast is desired to be captured (image capturing of a backlit subject or simultaneous image capturing of an indoor scene and an outdoor scene) using a solid-state image pickup device with an insufficient dynamic range, even if light quantity of exposure is controlled, overexposure may occur in a bright portion of the scene due to the saturation of the bright portion and underexposure may occur in a dark portion of the scene. As a result, both of these portions cannot be appropriately reproduced.
As a method of overcoming such a difficulty, Japanese Unexamined Patent Application Publication No. 6-141229 (corresponding U.S. Pat. No. 5,455,621) discloses a method of separately obtaining bright area information and dark area information by using two different electronic shutter speeds in a field or changing an electronic shutter speed for each field, and combining the obtained pieces of information into a single image. This method is employed by apparatuses capable of capturing an image with a wide dynamic range (wide dynamic range cameras). In addition, Japanese Unexamined Patent Application Publication Nos. 2004-120205 (corresponding patent No. US 2006/033823 A1) and 2002-84449 disclose apparatuses and methods for improving the quality of the above-described combined image.